1) Field of the Invention
This invention relates to an electrophotographic toner suitable for use in developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing and the like. More specifically, this invention relates to an electro-photographic toner capable of meeting requirements even for high-speed copying machines, while assuring well-balanced fixing and offset resistance and excellent grindability.
2) Description of the Related Art
Electrophotography as practiced in a PPC (plain paper copier) copying machine or printer generally comprises forming an electrostatic latent image on a photoconductor, developing the latent image with a toner, transferring the toner image onto a base sheet such as a paper sheet and then heating and fixing the toner image by a hot roll. Since fixing is conducted under heat and pressure, this process features high speed and excellent thermal efficiency, hence, superb fixing efficiency. This hot roll method is, in contrast to its excellent thermal efficiency, accompanied by the so-called offset phenomenon, that is, the problem that because the toner is brought into contact with a surface of the hot roll, the toner is stuck and transferred onto the surface of the hot roll and is then transferred back onto the next base sheet to smear the same.
On the other hand, there has been a steady demand toward high-speed copying machines. This has naturally resulted in a higher fixing roll speed, leading to a demand for a toner which requires only short-time heating for its fixing. From the viewpoint of energy saving or higher safety, there is also a strong desire for the development of a toner which can be fixed at a temperature as low as possible. To fix a toner at a low temperature, the toner is required to fuse at a temperature as low as possible and also to show excellent flowability in a fused state. To obtain a toner which fuses at such a low temperature and shows excellent flowability, it is necessary to lower the molecular weight of a resin to be employed. A reduction in molecular weight, however, leads to a resin having insufficient cohesive force, whereby the offset phenomenon tends to occur more readily. Such a low molecular weight is therefore not preferred. To cope with this problem, a low-molecular resin is generally blended with a high-molecular resin although the flowability is somewhat sacrificed, so that the low cohesive force of the low-molecular resin is supplemented by the cohesive force of the high-molecular resin. Examples of such techniques are proposed, for example, in Japanese Patent Publication Nos. 6895/1980 and 32180/1988, U.S. Pat. No. 4,921,771, etc. Such techniques are however still insufficient for the tendency toward higher copying speeds and, in many instances, countermeasures are taken based on improvements in copying machines. For example, silicone oil is coated on a surface of a hot roll by fabric or paper to prevent offsetting. In this case, the construction of the copying machine becomes complex so that its repair and maintenance are complicated, leading to higher repair and maintenance cost. This approach is hence not preferred. There is accordingly a demand for the development of a fixing toner, which is a toner for high-speed machines and is suitable for use in the oilless fixing method that does not require use of oil such as silicone oil.
In the development of toners for the oilless fixing method, numerous toners making use of a crosslinked polymer have been proposed as preventive measures for offsetting. For example, Japanese Patent Publication No. 36582/1985 discloses use of a crosslinked polymer produced by emulsion polymerization. In this case, the crosslinked polymer employed contains 50-99% of a gel component. As the content of such a gel component increases, offset resistance is improved but grindability is reduced. As the content of a crosslinked polymer component decreases, on the other hand, grindability is improved but offset resistance is not improved. It has hence been extremely difficult to satisfy both offset resistance and grindability. In addition, this method requires combined use of a dispersant or dispersion aid upon production of a crosslinked polymer. Such a dispersant or dispersion aid, however, is highly hygroscopic so that it adversely affects electrical properties, especially charge stability. It is therefore necessary to eliminate the dispersant or dispersion aid as much as possible after the production of the crosslinked polymer. A great deal of labor is however needed to completely eliminate the dispersant or dispersion aid by washing the resultant crosslinked polymer. This washing produces a lot of waste water, thereby posing a further difficulty in its treatment. U.S. Pat. No. 4,966,829 discloses to the effect that a good toner can be obtained by including a vinyl polymer which contains 0.1-60 wt. % of a gel component and, when analyzed by GPC in a form dissolved in THF, presents a main peak corresponding to molecular weights of 1,000-25,000 and at least one subpeak or shoulder corresponding to molecular weights of 3,000-150,000. However, the process adopted to produce the vinyl polymer is suspension polymerization which, like emulsion polymerization, also requires combined use of a dispersion or dispersing aid upon practice. The toner disclosed in this U.S. patent is therefore accompanied by exactly the same problem as the emulsion polymerization described above. With a view toward overcoming this problem, the present inventors have already provided, as a toner resin having good fixing property, a resin produced by solution polymerization (see U.S. Pat. No. 4,963,456).
A resin produced by solution polymerization requires elimination of a solvent subsequent to the completion of the polymerization. Since low-volatility components such as unreacted remaining monomers and decomposition products of an initiator can be all distilled off upon elimination of the solvent, it is possible to obtain a homogeneous resin which contains very little impurities and is stable electrically. The resin is therefore considered to be optimal for use in the production of a toner. Production of a crosslinked polymer by solution polymerization is however accompanied by the problem that the production cannot be continued due to occurrence of Weissenberg effect, that is, winding of the resin around a stirring shaft. The present inventors hence developed a process for achieving polymerization to a degree as high as possible in bulk or the like (see U.S. Pat. No. 5,084,368). A limitation is however imposed on the molecular weight available by the polymerization process, so that the offset problem has not been overcome fully. Further, Japanese Patent Publication No. 38700/1985 discloses a toner binder produced by heating and mixing (A) a copolymer containing 3-40% of a glycidyl-containing monomer and (B) a crosslinkable compound. The toner however contains many remaining epoxy groups so that toner particles of opposite charge are formed in a long-term test. The toner therefore involves a problem in durability. No fully satisfactory toner has been developed yet accordingly.